backing requires an authority or individual to provide guarantees which are not intrinsically given by the item exchanged. At the very least, all paper currencies require backing, because they are easy to counterfeit. This backing usually takes the form of military power and legal threats.
Nobody can be stopped from providing additionally guarantees for bitcoin. Thus to correct my previous post:
The question is misleading. Bitcoin doesn't haverequire backing. But other things may be backed by bitcoin.
E.g. a government can decide to participate in nation sponsored bitcoin mining. This is a reasonable thing to do, once a nation decides to back their paper currency with bitcoin.